1. Field of the Invention
The present invention relates to plasma processing apparatuses. More particularly, it relates to a plasma processing apparatus which, during plasma discharge, is capable of preventing drift of a discharge current that flows through a gap between electrodes of an electrode pair and further is capable of increasing the effective electrical power in a plasma space.
2. Description of the Related Art
FIG. 13 shows a typical plasma processing apparatus heretofore used for plasma treatment such as chemical vapor deposition (CVD), sputtering, dry etching, and ashing. The plasma processing apparatus in FIG. 13 has an electrode pair which includes a plasma excitation electrode 4 for exciting plasma and a susceptor electrode 8 facing the plasma excitation electrode 4. A workpiece W to be plasma-treated is placed between the plasma excitation electrode 4 and the susceptor electrode 8. The plasma excitation electrode 4 is connected to the power supply side of a radiofrequency (RF) generator 1 through an RF feeder 3 and a matching circuit 2A. The matching circuit 2A matches the impedance between the RF generator 1 and the plasma excitation electrode 4, and is accommodated in a chassis 120 made of a conductive material. The RF feeder 3 and the plasma excitation electrode 4 are enclosed in a housing 21, also made of a conductive material.
RF power transmitted from the output of the RF generator 1 is sequentially fed into the plasma excitation electrode 4 through the matching circuit 2A and the RF feeder 3. A shower plate 5 having many holes 7 is in contact with projections 4a on the bottom face of the plasma excitation electrode (cathode) 4. A gas inlet pipe 17 communicates with a space 6 formed between the plasma excitation electrode 4 and the shower plate 5. An insulator 17a is provided in the middle of the gas inlet pipe 17, which is made of a conductive material, to insulate the plasma excitation electrode 4 from the gas supply source. Gas from the gas inlet pipe 17 is introduced into a chamber 60 surrounded by a chamber wall 10 through the holes 7 of the shower plate 5. The upper side of the chamber wall 10 and the plasma excitation electrode 4 are hermetically sealed with an insulator 9 interposed therebetween.
The susceptor electrode 8 is provided in the chamber 60 and serves as the common (i.e., ground) side of the discharge voltage. A workpiece W, such as a wafer, is placed thereon. A shaft 13 supports the susceptor electrode 8. The lower portion of the shaft 13 and a chamber bottom 10A are hermetically sealed with conductive bellows 11. Air is exhausted from the chamber 60 by an exhaust system (not shown).
Since the susceptor electrode 8 can move vertically together with the shaft 13 and the bellows 11, the distance between the plasma excitation electrode 4 and the susceptor electrode 8 can be adjusted while maintaining a vacuum in the chamber 60. The lower portion of the shaft 13 is grounded, and the common side of the RF generator 1 is also grounded. The chamber wall 10 has the same DC potential as that of the shaft 13.
Referring to FIG. 14, a matching circuit 2A is provided between an RF generator 1 and an RF feeder 3. The matching circuit 2A includes a plurality of passive devices to achieve impedance matching between the RF generator 1 and a plasma excitation electrode 4 to correspond to changes in a plasma state in a chamber 60. Specifically, in the example shown in FIG. 14, the matching circuit 2A includes three passive devices, that is, a load capacitor 22 consisting of a vacuum variable capacitor, an inductance coil 23, and a tuning capacitor 24 consisting of an air variable capacitor. Only one inductance coil 23 is connected between the load capacitor 22 and the tuning capacitor 24.
In general, in the plasma processing apparatus described above, when drift occurs in a high-frequency current flowing through the gap between the plasma excitation electrode 4 and the susceptor electrode 8 during discharge, the plasma density within the plasma processing chamber 60 changes, resulting in a non-uniform plasma treatment of the workpiece W. Furthermore, the plasma processing apparatus described above has an additional disadvantage of large power loss in the matching circuit due to high parasitic RF resistance in the inductance coil in the matching circuit.
Accordingly, what is needed is an improved plasma processing apparatus which avoids the non-uniform plasma treatment.
Accordingly, embodiments of the present invention provide a plasma processing apparatus capable of performing uniform plasma treatment of the treatment surface of a workpiece.
Further, by lowering the parasitic RF resistance in an inductance coil of the matching circuit, embodiments of the present invention provide a plasma processing apparatus capable of increasing the plasma-processing capacity by reducing the power loss in a matching circuit and increasing the effective power in a plasma space.
The present invention, in its first aspect, provides a plasma processing apparatus having a plasma processing chamber that accommodates an electrode pair, the electrode pair including a plasma excitation electrode for exciting plasma and a susceptor electrode facing the plasma excitation electrode. A workpiece to be plasma-treated is placed between the electrodes. The plasma processing apparatus further includes a chassis that accommodates an impedance matching circuit, provided in the middle of a supply path for feeding RF power from an RF generator to the plasma excitation electrode. The impedance matching circuit functions matches the impedance between the RF generator and the plasma processing chamber. The chassis serves as a return path from the susceptor electrode to the RF generator. In the return path provided by the chassis, impedances are axisymmetrically equal at a predetermined frequency of the RF power with respect to the direction of a current returning to the RF generator.
While not wishing to be bound by any theory, it is believed that variation in plasma density in the plasma processing chamber is caused by drift in a high-frequency current flowing through the gap between the plasma excitation electrode and the susceptor electrode and that formation of a particular return current path in the chassis, which serves as a return path of the high-frequency current to the RF generator, causes the drift of the high-frequency current. In other words, the return current path particularly in the surface of the chassis is selectively formed along the portion having the lowest impedance at a predetermined frequency of the RF power used for plasma discharge. A discharge current flowing through the gap between the electrodes of the electrode pair varies in density such that the discharge current flows through the shortest path. Thus, making the impedances on the surface of the chassis axisymmetrically equal can prevent or suppress the drift in the gap between the electrodes of the electrode pair.
In accordance with embodiments of the present invention, the chassis may accommodate not only the impedance matching circuit but also other feeders such as an RF feeder from the impedance matching circuit to the plasma excitation electrode. The term xe2x80x9caxisymmetricxe2x80x9d mentioned above, which will be described below in detail, means not only the generally-defined state in which two points are disposed at equal distances from the central axis thereof on a straight line perpendicular to the central axis, but also the state in which a plurality of points are disposed at equal distances from the central axis thereof on a plane perpendicular to the central axis, the plurality of points also arranged with equal intervals between each other.
The cross-section of the chassis perpendicular to the central axis thereof preferably has a shape of a regular polygon or a circle.
When the quality of the material of the chassis is uniform with respect to the high-frequency current at a predetermined frequency, if the cross-section perpendicular to the central axis has a shape of a regular polygon or a circle, the impedances on the surface of the chassis are axisymmetrically equal with respect to the high-frequency current flowing along the central axis of the chassis. Therefore, the high-frequency current uniformly flows around the peripheral wall of the chassis, so that no deviated path is formed. Regular polygons mentioned above include, for example, not only a square or a regular hexagon, but also a regular triangle or a regular pentagon in accordance with the definition of xe2x80x9caxisymmetrialxe2x80x9d described above. The shape of the entire chassis is not limited to a regular polygonal prism or a cylinder. In accordance with embodiments of the present invention, the entire chassis may have a shape of a regular polygonal pyramid, a cone, a frustum of a regular polygonal pyramid, a frustum of a cone, a dome, or a combination of these shapes one on top of another and sharing one central axis.
When the quality of the material forming the chassis is not uniform with respect to the high-frequency current at a predetermined frequency or when the cross-section of the chassis does not have a shape of a regular polygon or a circle, the impedances can be adjusted so as to be axisymmetrically equal, for example, by providing slits or fins for adjusting the high-frequency current path in the chassis or by bonding a conductive component having different impedance to the chassis. That is adding or subtracting impedances, adjusting the high frequency current path, or both can be performed to achieve axisymmetry.
Preferably, the RF generator and the impedance matching circuit are connected with a coaxial cable wiring line extending from the center of the top of the chassis to the RF generator. It is also preferable that the plasma processing chamber be axisymmetrically formed and that the central axis thereof be coincident with the axis of symmetry of the impedances of the chassis. It is also preferable that the susceptor electrode be axisymmetrically formed and that the central axis thereof be coincident with the axis of symmetry of the impedances of the chassis.
The return path from the susceptor electrode to the RF generator is preferably formed such that the impedances at a predetermined frequency are axisymmetrically equal with respect to not only the chassis, but also the wiring line extending from the chassis to the RF generator; and, when the peripheral wall of the plasma processing chamber functions as the return path to the RF generator, also with respect to the peripheral wall of the plasma processing chamber and the susceptor electrode itself. It is also preferable that the wiring line extending from the chassis to the RF generator be a coaxial cable, the core of which is the outer path of the RF power and the shielding line of which is the inner path thereof, and that the wiring line extend from the center of the top of the chassis, that is, from a point coincident with the axis of symmetry of the impedances, to the RF generator. It is also preferable that the plasma processing chamber and the susceptor electrode be axisymmetrically formed and that respective central axes thereof be coincident with the axis of symmetry of the impedances of the chassis.
The impedances described above preferably have a specific value at the frequency of the RF power generated at the output of the RF generator.
In general, plasma treatment is performed at frequencies within the range from 13.56 MHz to 60 MHz. By adjusting the return path, such as a chassis, so that the impedances are axisymmetrically equal at a predetermined frequency in practical use within the above-mentioned range in the plasma processing apparatus, more uniform plasma treatment of the treatment surface of a workpiece, is achieved. Thus, the plasma processing apparatus is capable of preventing drift of the discharge current that flows through a gap between electrodes of an electrode pair during actual plasma discharge.
The present invention, in its second aspect, provides a plasma processing apparatus having a plasma processing chamber including an electrode for exciting plasma; an RF generator for supplying the electrode with RF power; and a matching circuit having an input terminal and an output terminal, for matching the impedance between the plasma processing chamber and the RF generator. The RF generator is connected to an RF supplier and the RF supplier is connected to the input terminal, the electrode is connected to an RF feeder and the RF feeder is connected to the output terminal, and a ground potential portion is connected between the input terminal and the output terminal. The matching circuit includes at least two inductance coils connected in parallel.
Such a structure of the plasma processing apparatus allows the parasitic RF resistance of the inductance coils in the matching circuit to be decreased, thereby reducing the power loss in the matching circuit.
In the plasma processing apparatus according to embodiments of the present invention, the inductance coils mentioned above are preferably arranged in parallel. It is also preferable that the inductance coils be axisymmetrically arranged with respect to the center of the electrodes.
Such a structure is preferable because the equilibrium state of the high-frequency current flowing through the inductance coils in the matching circuit is maintained, thereby reducing the parasitic RF resistance and achieving a stable plasma.
In the plasma processing apparatus according to embodiments of the present invention, the electrode for exciting plasma may also serve as an electrode of a tuning capacitor in the matching circuit. Thus, the apparatus is simplified and the parasitic RF resistance reduced.
Since the inductance coils of the matching circuit have a low parasitic RF resistance, low power loss is experienced in the matching circuit and increased effective power in the plasma space can be achieved.
The plasma processing apparatus according to the present invention is useful for efficiently performing plasma treatment such as CVD, sputtering, dry etching, or ashing.
The above and other objects, features, and advantages of the present invention will become clear from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.